Before the 2015 season, the Astros signed Luke Gregerson to be their closer for three years. The next winter, they traded a hefty package for Ken Giles, a younger, better pitcher who had recently taken over as the Phillies' closer. A few months into the 2016 season, they took the job from Gregerson, passed over Giles and gave it to Will Harris, an earlier waiver pickup who had outpitched them both. Giles took the job from Harris later that season, and he kept it throughout 2017, except that sometimes Chris Devenski, a breakout All-Star fireman, would get the saves instead of Giles or Harris or Gregerson. And then came the World Series, when manager A.J. Hinch seemed to have lost faith in the whole quartet. The closer for Game 7 of the 2017 World Series was the Astros' fourth- or fifth-best starter: Charlie Morton.

So, where do closers come from? Free agency, trades, waiver pickups, the farm, failed starters, good starters, bags of beans? In 2000, our own Christina Kahrl -- then at Baseball Prospectus -- asked just that question in a piece for ESPN. Most top starting pitchers began as younger starting pitchers, and most top hitters began as younger hitters. But top closers had little history in common, Kahrl found, other than being born and playing baseball.

Today, the same could be said -- though, as we'll get to, a lot has changed since Kahrl's survey. The pasts of closers, as a species, remain broad and inexplicable. And the immediate future of your favorite team's closer is only a coin-flip proposition.

1. Where closers come from, narrow picture

Who will close games for your favorite team 10 months from now? Odds are it won't be the name you have in mind. (Just consider the Houston closer!)

Of the 31 pitchers who served as their team's closer this past September -- we're giving both Mike Minor and Scott Alexander credit for sharing the job in Kansas City -- just 15 got even a single save for the same team in April. Only 12 got at least five saves for the same team in 2016. The median number of career saves, through 2016, for those 31 pitchers was just 19, about a half-season's worth. Four of our September closers had never saved a game before 2017, and 11 of them -- more than a third -- had no more than two career saves.

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It's hard, looking at that group now, to appreciate how weird a list of names it is. Of course, Hector Neris and Corey Knebel look like closers now, now that we've seen them do it and carried them for months on our fantasy rosters. But 12 months ago, Neris and Knebel had two career saves apiece. So let's try this: I'm going to predict the 30 pitchers who will be closers in September 2018, and you're going to look at me like I'm insane:

There you go. You're looking at me like I'm insane, and of course I am! There are two names on there I just learned five minutes ago. There are pitchers who haven't pitched in the majors in more than a year, who have never pitched in relief, who have no saves, who are currently behind legit relief aces, who are practically punch lines, who are probably shocked that this article set off a Google Alert for their name. But each of those pitchers is in, roughly, the same career state one of our September 2017 closers was in 12 months ago. For example:

Henderson Alvarez, an above-average starter four years ago, missed almost all of the past three seasons with shoulder injuries. He is in his late 20s, and has never made a relief appearance. Just like Mike Minor, an above-average starter four years ago, who missed all of two seasons with injuries before 2017. He was in his late-20s and had made one relief appearance (as a rookie). In this résumé, Kansas City found an excellent reliever who would serve as closer (with near perfection) in September.

Alan Busenitz was a 25th-round pick, never a prospect, used almost exclusively as a reliever in the minors but without any real flash. He doesn't strike many guys out, but he had a low ERA in low-leverage innings for Minnesota this year. Just like Alex Claudio, a 27th-round pick, never a prospect, used almost exclusively as a reliever in the minors but without any real flash. Claudio doesn't strike many guys out, but he had a low ERA in low-leverage innings for Texas in 2016. In this résumé, Texas found its second-half closer in 2017.

Tom Koehler is one of the least interesting starters in baseball, a veteran innings-eater with years of high ERAs, high walk rates and (despite throwing hard) low strikeout rates. Just like Juan Nicasio, who entered the 2017 season with a career ERA of 5.11 as a starter and ended it closing games in a pennant race.

Of course, Alvarez and Busenitz and Koehler and a dozen other names on that list are extremely unlikely to close next September. But guys like them will. Guys you're not thinking of at all right now. You really can't overstate the unlikelihood of "Los Angeles Angels closer Blake Parker."

There are a lot of closers and quasi-closers on the free-agent market this year: Davis and Greg Holland at the top, but also Addison Reed, Brandon Morrow, Jake McGee, Tony Watson, Joe Smith, Fernando Rodney, Steve Cishek, Minor and Nicasio. The September survey has some implications for them this winter as they try to sell their services: It means each and every one of them can sell himself as a closer. Not necessarily as the Opening Day closer, or even the midseason closer, but as one of the many pitchers who will be in the mix when the team turns to plan B, C and D. For all but a small handful of teams, more than one closer is going to be needed this year.

2. Where closers come from -- big picture

Seventeen years ago, Kahrl looked at the origins of every pitcher who had saved at least 30 games in the previous decade. A very small handful (8 percent) had been relievers from their first day in professional baseball; a slightly larger handful (18 percent) had been converted to relief after just a handful of low-minors starts.

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"That leaves 36 of our 49 closers -- almost three-quarters of the group -- who made their way through the minors as starting pitchers, moved to closing because they struggled as starters or eventually fell into the role," Kahrl wrote. "While it's undoubtedly self-serving to lump Dennis Eckersley, a closer who had enjoyed a successful major-league career as a starter before turning to closing, with guys like Roberto Hernandez or Rod Beck or Steve Farr or Mike Schooler, the fact is these guys were all starting in Double-A and Triple-A and even the majors, and the expectation and the organizational goal for each of them was that they were going to be starting pitchers."

This idea -- replicated, to some degree, a few years later by Nate Silver at Baseball Prospectus -- has informed a lot of thinking about top relievers. They're guys who weren't good enough to start, who even tried -- and tried and tried -- to start, but failed and failed. "Baseball's loser's bracket," I've called it.

But that view is probably a little outdated. I repeated Kahrl's process with every pitcher who has saved at least 30 games in a season in the 2010s. There are 66 of them, ranging from elite, Hall of Fame-bound closers (Kimbrel, seven times) to forgettable, one-year successes (Shawn Tolleson).

Sixteen of those 66 -- 24 percent -- never made a single minor league start. Some 40 percent made 20 or fewer minor league starts, and just about half never started at any level higher than high-A. (In the lowest minors, there's often little distinction between starters and relievers.) Two-thirds have never made a major league start, and 80 percent have made 10 or fewer starts in the majors. And of the converted starters, a notable handful -- Neftali Feliz, Trevor Rosenthal, Aroldis Chapman and Osuna -- were moved to the bullpen before they had failed as starters.

Put it this way: Of our 66 Proven Closers, there are more who were converted position players (seven) than who started at least 20 games in the majors. The league's ninth innings are no longer staffed primarily by failed starters, even failed minor league starters. Far more future relief aces are making their pro debuts as relievers, or else committing to relief work almost immediately.

There are a lot of plausible explanations for this subtle shift -- including better player development and better, earlier identification of relief "types" in the low minors -- but most come down to incentives. Because starting pitchers throw fewer innings than they did 20 years ago, and are much more likely to spend a big hunk of their early experience being injured, there's less incentive to try to force every pitcher into the starting-pitcher box. And because relievers collectively take on more innings, get paid more and increasingly dominate the postseason, there's more incentive to develop high-leverage relievers. Further, putting pitchers on the reliever track gets them to the majors faster and makes use of their limited window of good health rather than wasting "bullets" in the minors.

It's subtle, but it's a step that might someday look significant. "Reliever" is no longer what teams settle for out of a pitcher's career, or a dirty word to put on a prospect. It's increasingly considered a desirable outcome. This lines up with what baseball has been trending toward, with little sign of abatement: a game dominated by relievers who, as a group, might someday provide more collective value than starters do.